A steampunk novel or story is a hard vision to capture if one wants to do it just right.  It involves a certain type of setting and feeling, but also must combine elements of science, science fiction, and fantasy.

First thing you are going to need is your story idea.  Is it going to be a character driven store that just uses a steampunk theme as its vehicle?  Or is there something to the plot that can only be done in a steampunk setting?

Next layout your characters.  Don't just make them on your first thought.  Write down the information about the character, what makes him or her unique, what steampunk role do they fit, and how you can make them unique.  Are they a scientist or explorer?  Maybe a steam engine mechanic?  Once you found their role, what is different about the character?  Maybe they are socially inept in public, but are an idol in an underground boxing league because of their fighting skill.  Maybe the heroine is an ugly duckling, but it is only because she rather be making flying machines or smithing metal, than gossiping with the girls and fixing herself pretty.

Once you have a setting and characters, hopefully by this time you also have a plot.  Take structure into mind here.  A story has a beginning, middle, and end.  It has an inciting incident that triggers the whole conflict of the story, and is related to the climax, where hopefully the inciting incident is resolved.  You need to break up your beats into scenes, your scenes into series, and your series into acts. Finally your acts make up your story.

Next is research, but don't get too involved.  You just need to study the London Victoria time period and get a grasp for how people spoke in that time period, what things were called, etc.

Now you have to write.  It is the hardest part of the whole thing, but you have the hard work done, now do it!